Call a lawyer. Now. As I understand it, you are open to prosecution for possession of a short-barrelled rifle while not also having the proper paperwork from BATFE.
This is not just a matter of retaining a family heirloom; this is a matter of keeping you from being prosecuted. The BATFE is not an organization upon whose benevolence one wants to rely. All it takes is a ***** agent and you could be dealing with legal trouble.
Then, ask a moderator to delete this thread so its not hanging out here for everybody in the world to stumble upon, including the government.
Well, actually, before calling the lawyer, you might want to see if you can verify that the barrel is indeed a length that requires BATFE paperwork. This could be a gunstore screw-up.